Extreme-ultraviolet synthesis of nanojet-like ejections due to coalescing flux ropes

TL;DR

This study tackles the challenge of diagnosing nanojets in the solar corona by producing synthetic EUV observables for nanojet-like ejections arising from the coalescence of flux ropes, based on a 2.5D resistive-MHD model implemented with MPI-AMRVAC. Forward modeling yields emissivity maps and spectral profiles for SDO/AIA and the upcoming MUSE instrument, revealing a localized, bi-directional nanojet with lifetimes around 20 s, sizes up to a few Mm, and Doppler shifts near 100 km s^-1. The synthetic signatures show strong concordance with observed nanojets in hot EUV channels and offer detailed diagnostics of temperature, density, and energetics, including plasmoid dynamics. The work provides a bridge between MHD simulations and observations, offering testable predictions for current and future solar missions and contributing to our understanding of nanojets and their potential role in coronal heating.

Abstract

Detection and characterization of small-scale energetic events such as nanoflares and nanojets remain challenging owing to their short lifetimes, small spatial extent, and relatively low energy release, despite their potential role in coronal heating. Recent observations have identified nanojets as small-scale (length $\lesssim 6.6$~Mm, width $\lesssim 1$~Mm), fast ($\sim$~few 100 km s$^{-1}$), and short-lived ($\lesssim 30$~s) ejections associated with nanoflare-scale energies, providing evidence of magnetic reconnection at small spatial scales. However, the lack of synthetic diagnostics has limited the connection between magnetohydrodynamic (MHD) models and observations. In this Letter, we present synthetic observations of the coalescence of two flux ropes, leading to nanojet-like signatures from a numerical model obtained with the \texttt{MPI-AMRVAC} code. We report synthetic observables in Extreme-ultraviolet lines compatible with existing instruments such as SDO/AIA, and upcoming MUSE mission, and compare the synthetic observables with an existing observation of nanojets. The synthetic diagnostics of the emissivity maps, Doppler velocity, thermal, and non-thermal line broadening produce key observational properties, suggesting a plausible 3D scenario for nanojet generation where tiny flux ropes reconnect within loops. Our results provide predictions for the detectability of nanojets with current and future spectroscopic facilities, and establish a bridge between MHD modeling and observations.

Figures (6)

• Figure 1: Spatial variations of the plasma density ($\rho$), temperature ($T$), and horizontal velocity ($v_x$) are shown from left to right in each column, as obtained from the MHD simulation by SMI25. The temporal evolution of these quantities at $t = 118.02$, $128.81$, and $145.98$ s is presented in the top, middle, and bottom panels, respectively. The nanojet-like bi-directional outflows are apparent in the $v_x$ maps within the dashed ellipses in the top and middle panels, whereas these flows almost disappear within the marked ellipse in the bottom panel. The horizontal dashed line in the top-left panel denotes the $y$-level at 27.2 Mm. An animation of the time evolution is available online.
• Figure 2: Top row: Synthetic emissivity maps in various SDO/AIA broadband channels at the spatial resolution of the simulation at $t = 118.02$ s. The emissivity values, given in DN cm$^{-1}$ s$^{-1}$ pix$^{-1}$, are indicated by the corresponding color bars. The black dashed line in the left panel marks the same location as in the top-left panel of Fig. \ref{['fig:MHD']}. Bottom row: Same as the top row, but degraded to a spatial resolution of $1".2$ comparable to AIA resolution, where the marked green arrows show the location of the nanojet-like feature. An animation of the time evolution is available online.
• Figure 3: Top row: synthetic emissivity maps in different MUSE lines with the spatial resolution of the simulation at $t=118.02$ s, where the horizontal dashed line is a marker at $y=27.2$ Mm. Bottom row: same as the top row, but with a resolution of $0".4$, and $0".167$ along the $x$ and $y$ directions respectively, compatible to MUSE.
• Figure 4: Top row: Spectral maps in different emission lines at $t=118.02$ s, where the intensities are calculated by integrating along the $x$ direction. The white boxes are placed at around $y=27.2$ Mm to highlight spectral signature of the nanojet-like ejections. Middle row: The left panel represents spectral profiles for the emission lines (marked with the corresponding legends) at $y=27.2$ Mm, and the right panel shows the variation of total line width along the $y$ direction, where the vertical dashed line is a marker at $y=27.2$ Mm. Bottom row: Time-distance map of the intensities, which are obtained by integrating along the slits, which are placed horizontally (along $x$) at 31 different $y$-levels. The green ellipse in the left panel shows the region with brightening between $y\approx 27$ and 30 Mm and $t\approx 100$ and 120 s, showcasing the nanojet-like signature. The maps at the middle and right panels highlight the evolution of the merging plasmoids (as marked in the middle panel).
• Figure 5: Top row: Selected AIA snapshots of nanojet N2 from Ramada:2022, with arrows marking the plasmoid-like structures P1 and P2 in the strands moving towards each other. The white solid line is a path taken to produce the time-distance diagrams, where the value for a given point along the path is obtained by averaging eight values spaced evenly along its normal line between the dashed lines. The white vertical dashed line marks the timestamp of the images, where $T_0$ is at 19:36:18 UT. An animated version of this figure is available online.